1. Field of the Invention
This invention relates to a method of producing a magnetic recording medium having a multi-layer structure.
2. Description of the Prior Art
A magnetic recording layer for a magnetic recording medium of multi-layer structure which has high high-frequency performance and tremendously improved recording sensitivity has been broadly used. There is always a desire to improve the material used in this kind of magnetic recording medium and in the method of orientation of the magnetic material as well as in the electromagnetic performance thereof. A video tape is an example of one of the multiple-structure recording media used for short-wavelength recording and in which the video signals are recorded in the surface region of the magnetic layer. Therefore, it is necessary to smooth the surface of the magnetic layer and enhance the residual magnetic flux density in the video tape.
Further, it is also desired to lower the manufacturing cost of the recording medium as well as improve the performance such as the density or S/N ratio. Therefore, it has been practiced to smooth the surface of the coating layer by use of a super-calender roll at a higher temperature and a higher pressure as a surface treatment to smooth the surface of the magnetic layer of a video tape and to increase the residual magnetic flux density. It is also known to finely divide the magnetic powder to be kneaded for the same purpose.
However, in this kind of surface treatment, it has not been possible to make a magnetic layer having both the surface smoothness and the high residual magnetic flux density as desired as well as a magnetic recording medium for video recording at a low manufacturing cost.
The magnetic layer is prepared by kneading an organic solvent containing magnetic powder and a binder to form a magnetic coating material and applying the magnetic coating material on a substrate. At the time of kneading, the magnetic coating material is subjected to high shear force to increase the residual magnetic flux density. However, the magnetic coating material obtained by kneading under a high shear force is likely to have a rough surface when it is applied on the substrate. Therefore, it has been difficult to obtain a magnetic layer having a smooth surface and a high residual magnetic flux density.
Therefore, it has been proposed to make a double-structured magnetic layer having an upper layer and lower layer containing different residual magnetic flux densities (Br). For example, it has been known to provide an upper layer of high residual magnetic flux density filled with iron nitride having a high residual magnetic flux density (Br) and on a lower layer having a high surface smoothness filled with less iron oxide containing cobalt. However, it is not desirable from the viewpoint of manufacturing to use the different kinds of ferromagnetic material in the two magnetic layers, which will raise the manufacturing cost. Therefore, it has also been proposed to use the same ferromagnetic material for the two magnetic layers but changing the method of making the magnetic coating material for the two layers, wherein the magnetic coating material used for the lower layer is made by kneading an organic solvent containing magnetic powder and a binder with a kneader of low shear force, and then dispersing it in a wet medium dispersion mixer, and the magnetic coating material used for the upper layer is made by kneading the same organic solvent as the solvent for the lower layer with a kneader of high shear force, and then dispersing it in a wet medium dispersion mixer. The kneader having a high shear force is known, as a two-shaft continuous kneading and mixing machine which is able to knead at a high filling rate, and the kneader having a low shear force is known as a batch type double arm kneader or the like.
In the above-described double-layer structured magnetic recording layer, the lower layer is desired to have as high a filling rate as possible using a magnetic body of high magnetization rate so as to be suited for long wavelength recording having a large recording depth; and the upper layer is desired to have as high a filling rate as possible using finely divided ferromagnetic powder having high coercive force so as to lower the noise level and to be suited for short wavelength recording having small recording depth. The upper layer is, however, different from the lower layer, because it is brought into direct contact with the recording head or the tape guide in the recording deck. Accordingly, a high surface smoothness is not desired because if the material is kneaded too much and dispersed to obtain a high filling rate, the surface of the magnetic layer is smoothed and lowers the sliding performance while it improves the electromagnetic conversion characteristics. Further, the finely divided magnetic body is likely to be deformed in the course of kneading to lower the magnetic characteristics. That is, both the upper and lower layers must be subject to the appropriate manufacturing processes, respectively, so that, the purpose of making the double-layer structured magnetic layer is to provide high magnetic characteristics and enhanced running durability of the magnetic recording medium.